A wealthy geneticist asks a number of US scientists to raise bonobo chimps in their homes, supposedly to study their reactions to enriched intellectual environments. But from the moment she arrives in Dr. Jim Dutton's home, Umber is much more than a project animal. Different than any chimp he has ever known, Dutton marvels as Umber masters sign language, learns to use a computer and becomes a "sister" to his daughter, Brett. When he compares notes with other host families, he finds that all of the chimps seem to be equally exceptional, and he begins to wonder exactly how - and why - the animals have been altered. But it isn't until the chimps' owner begins recalling the bonobos that Dutton sets out in pursuit of the truth.